The term filter in this context relates to a filter medium that is thin relative to its length and width, for example a woven material, a warp knitted material, a knitted material or a fleece whose fibers are for example made from metal, plastic or glass. The enumerations are exemplary and not complete. The filter can be flat or have a three dimensional shape, it can be for example zigzag shaped, meander shaped, cylinder or frustum shaped or two dimensionally cambered.
The filter includes a preferably linear gasket, in particular along an edge of the filter, wherein the gasket can also be flat. The edge can be an outer edge, or when the filter includes one or plural holes, an inner edge. The gasket can also extend within the filter at a distance from edges of the filter. The gasket is made in particular from an engineered plastic material with long term elasticity, for example an elastomeric material, a hard plastic material, a polyamide, a polyester, for example fluor silicone for a an oil filter or for example polypropylene as a carrier and not necessarily as a filter of a membrane for a fuel cell. Thus, the enumeration is exemplary and not complete. The gasket is in particular a solid material, thus not a foam, this means it does not include any intentional pores, gas inclusions, voids or similar cavities.
Instead of the gasket or in addition to the gasket the filter can include a frame which provides additional stability to the filter.
It is known for example to glue on or otherwise apply a prefabricated gasket to a filter. It is also possible to apply a gasket at a filter through injection molding, wherein the filter is inserted into an injection mold. Injection molding requires an injection molding tool which includes a shape of a seal to be produced as an empty space which is designated as a cavity. In order for a plasticized compound to completely fill the cavity during injection molding a sufficient cross section of the cavity is required which is also a function of the flow length of the cavity. A height of 400 μm of the cavity is considered as a lower practical limit for plastic injection molding today in case surfaces which come in contact with the plasticized plastic material during injection molding are smooth. In case there is a woven material, a fleece or a similar filter medium in the cavity, the cavity has to be substantially taller due to the brake effect of the filter medium, at least two to three times the height of the cavity is required. The filter medium does not only slow down the plasticized plastic material that is injected into the cavity under pressure, but the flowing plastic material pushes fibers of the filter medium in front of itself and squashes the fibers so that they block the cavity.